A 1,000-watt microwave oven heats soup. A 0.001-watt dental X-ray needs a lead apron.
We live in a bath of invisible waves. Radio carries signals to your phone, microwaves warm food, and ultraviolet light from the Sun can burn your skin. The word “radiation” often sounds like danger. But power is not what makes some radiation harmful. The energy of each single photon decides. This page shows how to judge a claim about radiation.
Core Claims
- Photon energy. Light comes in packets called photons. Higher frequency means more energy per photon.
- What each band does. Microwaves make molecules rotate. Infrared makes bonds vibrate. Visible light moves electrons. Ultraviolet, X-rays and gamma rays can break bonds.
- Ionizing radiation. Each photon of ionizing radiation can knock electrons off atoms. Non-ionizing radiation cannot.
- Brightness is not energy. A brighter source sends more photons, not stronger ones. Many weak photons cannot add up to break a bond.
Energy per photon rises with frequency
Retrieval Checklist
- Say how photon energy changes as frequency rises.
- Match each band of the spectrum to what it does to matter.
- Say which radiation can damage living cells, and why.
- Judge a claim by asking about photon energy, not power.
Light travels as photons, and higher frequency means more energy per photon.
All electromagnetic radiation moves through space at the speed of light. It acts as a wave and as a stream of tiny packets of energy. Each packet is a photon.
A photon’s energy depends on its frequency. A high-frequency photon has more energy than a low-frequency photon. Frequency and wavelength trade off, so a short wavelength means a high frequency.
The energy of one photon depends only on its frequency. A brighter source sends more photons. It does not make any single photon stronger.
Thinking a brighter or more powerful source has stronger photons. Power counts all the photons together. Each photon still carries the energy of its own frequency.
What a photon does to matter depends on its energy.
When radiation strikes matter, the matter absorbs the photons. What happens next depends on the energy of each photon.
1. Microwaves make molecules rotate
Microwave photons have little energy. They twist polar molecules such as water back and forth. The twisting turns into heat as molecules rub, and that warms your food. These photons cannot break a bond.
2. Infrared makes bonds vibrate
Infrared photons make bonds stretch and bend. This is how greenhouse gases such as CO2, CH4 and H2O absorb heat that Earth radiates.
3. Visible light moves electrons
Visible photons lift outer electrons to higher energy levels. In plants, this drives photosynthesis. In your retina, it starts the signal for vision.
4. Ultraviolet, X-rays and gamma rays can ionize
These photons carry enough energy to knock electrons off atoms and to break bonds. This is ionizing radiation. In living cells it can break DNA, which can cause mutations, cell death and cancer.
Thinking all radiation damages cells. Radio waves, microwaves, infrared and visible light cannot break bonds. A strong source can still heat and burn tissue, but it does not ionize it.
Change the band and the brightness, and watch the target.
Pick a band, a brightness and a target. Each photon works alone, so look at what one photon can do before you look at how many arrive.
Why can X-rays damage cells when radio waves cannot?
The explanation appears after you predict.
- A photon of radio, microwave, infrared or visible light
- A photon of ultraviolet, X-ray or gamma radiation
- The source (left) and the target (right)
- The bond-breaking level on the energy bar
- Brighten a weak band. Load radio waves at brightness 5 and press Record. Look for: 16 photons each second, and 0 bonds broken. A brighter source sends more photons, not stronger ones.
- Dim a strong band. Load X-rays at brightness 1 and press Record. Look for: 1 photon each second, and 1 bond broken each second. One X-ray photon is enough.
- Cross the bond-breaking level. Load visible light and press Record, then load ultraviolet and press Record. Look for: visible light breaks 0 bonds each second. Ultraviolet breaks 4, because its bar rises above the dashed line.
Use your evidence
To judge a claim about radiation, ask about the photons.
Claims that Wi-Fi, 5G towers or cell phones cause cancer spread widely online. How does a scientist judge a claim like this in a published source?
Ask three questions. Which band does the source use? Does one photon carry enough energy to break a bond? Does the claim confuse power with photon energy?
A bond cannot save up energy from many weak photons. One photon breaks one bond, or none. A radio tower sends huge numbers of photons, but every one of them is too weak to break a DNA bond.
A strong enough transmitter can warm tissue and even burn it. That is a heating effect, and it is different from damage to DNA.
Thinking a stronger transmitter must be more dangerous to DNA. Power sets how much heat a source makes. Photon energy sets whether a bond can break.
E = hf, energies in eV, and a comparison of three devices.
EnrichmentCalculating photon energy, and what happens to DNA
Extra depth. The rest of the lesson works without it.
Photon energy
Light of frequency f and wavelength λ travels at the speed of light c. A photon’s energy E follows from Planck’s constant h = 6.626 × 10−34 J·s.
Scientists often give photon energy in electron-volts (eV). One eV is a tiny unit of energy. A typical covalent bond in DNA needs about 3 to 4 eV to break. Photons above about 10 eV can ionize atoms.
| Band | Example | Photon energy |
|---|---|---|
| Radio | 1 MHz | 4.1 × 10−9 eV |
| Microwave | 2.45 GHz | 1.0 × 10−5 eV |
| Infrared | 30 THz | 0.124 eV |
| Visible | 550 nm | 2.25 eV |
| Ultraviolet | 300 nm | 4.13 eV |
| X-ray | 0.1 nm | 12,400 eV |
| Gamma ray | 0.001 nm | 1,240,000 eV |
A 5G photon at 28 GHz carries 0.00012 eV. That is about 30,000 times too weak to break a DNA bond.
What ionizing radiation does to DNA
An ionizing photon can break the covalent bonds of the DNA backbone. This can cut both strands. It also makes hydroxyl free radicals, which attack nearby molecules. Cells that repair this damage wrongly can mutate.
A committee reviews three devices. Device 1: a 5G base station, 28 GHz, photon energy 0.00012 eV, 500 W. Device 2: a tanning bed, λ = 300 nm, photon energy 4.13 eV, 100 W. Device 3: a dental X-ray, λ = 0.1 nm, photon energy 12,400 eV, 0.001 W in short pulses.
(a) Compare the photon energy of Device 1 with Device 2. Say what each photon does to matter. [1 mark]
(b) A forum says the 500 W tower is more dangerous than the 100 W tanning bed. Use the numbers to judge the claim. [2 marks]
(c) Mineral sunscreens use zinc oxide or titanium dioxide particles. Say how they protect DNA. [1 mark]
- Comparison (1 mark): You say 5G photons only warm matter, and 4.13 eV photons can break bonds.
- Power or photons (1 mark): You say the claim confuses total power with the energy of one photon.
- No pooling (1 mark): You say weak photons cannot add up to break a bond, however many arrive.
- Sunscreen (1 mark): You say the particles absorb ultraviolet photons and turn the energy into heat.
Fill the blanks from memory.
Type each blank from memory, then press Check. Press Show answers after your first check if you are stuck.
1. The energy of one photon rises as its rises.
2. Microwave photons make polar molecules such as water , which warms food.
3. Infrared photons make chemical bonds stretch, bend and .
4. Ultraviolet, X-ray and gamma photons are radiation because they can knock electrons off atoms.
5. A brighter source sends more photons, but each photon has the same .
An advertisement says: “5G towers give off dangerous radiation that breaks the DNA in your brain cells. Buy our sticker to block it.”
Judge this claim. Use photon energy and the difference between ionizing and non-ionizing radiation.
Name the band that 5G uses. Say what one photon of that band can and cannot do. Write your answer first, then compare it with the model answer.
5G uses radio waves. Radio photons carry far too little energy to break a bond in DNA. Radio is non-ionizing radiation.
The advertisement may be thinking about power. But a bond cannot save up energy from many weak photons, so a stronger signal does not change the result. At most, a strong source warms tissue. The claim is not supported.
Write your answer first. Then grade yourself.
Give yourself 1 mark for each idea you wrote down.
A health committee reviews three devices. Device 1 is a cell tower that sends radio waves at 500 W. Device 2 is a tanning bed that sends ultraviolet light at 100 W. Device 3 is a dental X-ray machine that sends X-rays at 0.001 W in short pulses.
(a) Rank the three devices by the energy of one photon, from lowest to highest. [1 mark]
(b) A forum says the 500 W tower is more dangerous than the 100 W tanning bed because it has more power. Judge the claim. [2 marks]
(c) Say why the dental X-ray needs a lead apron even though it has the lowest power. [1 mark]
- Ranking (1 mark): You rank the tower (radio), then the tanning bed (ultraviolet), then the X-ray machine.
- Power or photons (1 mark): You say the claim confuses total power with the energy of one photon.
- No pooling (1 mark): You say radio photons are too weak to break bonds, however many arrive.
- X-ray (1 mark): You say X-ray photons are ionizing, so even a few can damage cells.
Mineral sunscreens contain tiny particles of zinc oxide or titanium dioxide. The particles absorb ultraviolet photons before they reach your skin. They turn the photon energy into harmless heat, so the DNA below stays whole.
Adaptive Competency Practice: HS-PS4-4
Test your understanding of radiation and matter one question at a time. The adaptive engine covers photon energy, what each band does to matter, ionizing radiation, and judging claims.